Contrib Mineral Petrol (1980) 75:407-414 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1980 The Thermodynamic Regime of Metamorphism in the Ancient Subduction Zones L.L. Perchuk and L. Ya. Aranovich Institute of Experimental Mineralogy, Academy of Sciences of the USSR, 142432 Chernogolovka, Moscow District, USSR Abstract. Based on mineralogical themometry and barometry and computation of mineral reactions modelling metamorphic sequence, a geotherm for met- amorphic belts of the subduction zones has been deduced. Relatively low PT-values (3 kbar/200~ C) correspond to zeolite and prehnite-pumpellyite recta- sediments and at higher pressures and temperatures (10kbar/400 ~ lawsonite-glaucophane assemblages become unstable. The PT-curve achieves maximum at 11 kbar and 470~ to drop down to normal geo- therm (Perchuk 1977). High concentration of H20 in the metamorphic fluid has been revealed, the differ- ence between Pfl and PH2o being less than 2 kbar. Consideration has also been given to specific thermo- dynamic regime of zeolite and prehnite-pumpellyite zones of the younger island arcs, where lawsonite- glaucophane zones are absent. Here the geotherm has been found to rise from 0.2kbar/120~ up to 4 kbar/350 ~ C and P~2o-regime similar to that of glau- cophane schists formations. Introduction The metamorphism of the subduction zones is be- lieved to have taken place at an early stage of the evolution of geosyncline complexes ("protogeosyn- clines') at the continental boundaries and at the beginning of island-arc development, Both zoning and the geological position of zones within the general tectonic structures have been established accurately by petrologic and geological methods (Ernst 1971; Miyashiro 1973; Dobretsov 1974; Perchuk 1973 and others). Both the geothermic and barometric determi- nations (Perchuk 1973, 1977) of mineral equilibria in Reprint requests to: L.L. Perchuk 72 / /// G ~a ~o .I f Z 1t I I I I I ZOO 400 gOO T, *C Fig. 1. Variation of temperature with pressure in glaucophane schist provinces (curve 1) and the ancient geothermal gradient in the developed metamorphic areas (curve 2) according to Perchuk (1977, p. 336) the zones have revealed certain P und T distribution patterns across the zoning, the maximum P being at 4000-450 ~C (Fig. 1). The right-hand part of the curve in Fig. 1 is based on numerous PT-measurements for natural assemblages. The purpose of this paper is to ascertain the shape of the left side of the curve. Methods of Calculations The calculations were based on the gas-mineral re- actions involving CO 2 and H20 at low temperatures. The minerals and their thermodynamic properties are given in Table 1. Data for Ab, An, Arg, Jd, Chl, Qz, Cal, Tr, H20, and CO/ were taken from Karpov's compilation (Karpov et al. 1976), the H20 fugacity coefficients (7~) for high T and P were from Burnham et al. (1969) and those for CO 2 from Perchuk (1977). The zoisite free energy and entropy (Aranovich 1977; Perchuk and Aranovich 1979) are in good agreement 0010-7999/80/0075/0407/$01.60